Glucose

The blood glucose test may be used to:

• Detect high blood glucose (hyperglycemia) and low blood glucose (hypoglycemia)
• Screen for diabetes in people who are at risk before signs and symptoms are apparent; in some cases, there may be no early signs or symptoms of diabetes. Screening can therefore be useful in helping to identify it and allowing for treatment before the condition worsens or complications arise.
• Help diagnose diabetes, prediabetes and gestational diabetes
• Monitor glucose levels in people diagnosed with diabetes

A few different testing protocols may be used to evaluate blood glucose levels, depending on the purpose.

Screening and Diagnosis
The following tests may be used for screening and diagnosis of type 1, type 2 or prediabetes. (Gestational diabetes testing is different—see below.) If the initial screening result from one of the tests is abnormal, the test is repeated on another day. The repeat result must also be abnormal to confirm a diagnosis of diabetes.

• Fasting glucose (fasting blood glucose, FBG) – this test measures the level of glucose in the blood after fasting for at least 8 hours.
• 2-hour glucose tolerance test (GTT) – for this test, the person has a fasting glucose test done (see above), then drinks a 75-gram glucose drink. Another blood sample is drawn 2 hours after the glucose drink. This protocol "challenges" the person's body to process the glucose. Normally, the blood glucose level rises after the drink and stimulates the pancreas to release insulin into the bloodstream. Insulin allows the glucose to be taken up by cells. As time passes, the blood glucose level is expected to decrease again. When a person is unable to produce enough insulin, or if the body's cells are resistant to its effects (insulin resistance), then less glucose is transported from the blood into cells and the blood glucose level remains high.
• A different test called hemoglobin A1c may be used as an alternative to glucose testing for screening and diagnosis. (For more, see the article on A1c.).

Sometimes a blood sample may be drawn and glucose measured when a person has not been fasting, for example, when a comprehensive metabolic panel (CMP) is performed. If the result is abnormal, it is typically followed up with a fasting blood glucose test or a GTT.

Glucose blood tests are also used to screen pregnant women for gestational diabetes between their 24th and 28th week of pregnancy. The American Diabetes Association and the U.S. Preventive Services Task Force recommend that pregnant women not previously known to have diabetes be screened and diagnosed, using either a one-step or two-step approach. The American College of Obstetricians and Gynecologists (ACOG) recommends the two-step approach.

• One-step 2-hour oral glucose tolerance test (OGTT). After a fasting glucose level is measured, a woman is given a 75-gram dose of glucose to drink and her glucose levels are measured at 1 hour and 2 hours after the dose. Only one of the values needs to be above a cutoff value for diagnosis.
• Two-step
  • Perform a glucose challenge test as a screen: a woman is given a 50-gram glucose dose to drink and her blood glucose level is measured after 1 hour.
  • If the challenge test is abnormal, perform a 3-hour oral glucose tolerance test. After a woman's fasting glucose level is measured, she is given a 100-gram glucose dose and her glucose is measured at timed intervals. If at least two of the glucose levels at fasting, 1 hour, 2 hour, or 3 hour are above a certain level, then a diagnosis of gestational diabetes is made.

Glucose testing is also used to test women who were diagnosed with gestational diabetes 6-12 weeks after they have delivered their baby to detect persistent diabetes.

Monitoring
Diabetics must monitor their own blood glucose levels, often several times a day, to determine how far above or below normal their glucose is and to determine what oral medications or insulin(s) they may need. This is usually done by placing a drop of blood from a skin prick onto a glucose strip and then inserting the strip into a glucose meter, a small machine that provides a digital readout of the blood glucose level.

Urine
Urine glucose is one of the substances tested when a urinalysis is performed. A urinalysis may be done routinely as part of a physical or prenatal checkup. The health practitioner may follow up an elevated urine glucose test with blood glucose testing. Urine glucose testing is a screening tool, but it is not sensitive enough for diagnosis or monitoring.

Other tests, such as diabetes autoantibodies, insulin, and C-peptide, may sometimes be performed along with these tests to help determine the cause of abnormal glucose levels, to distinguish between type 1 and type 2 diabetes, and to evaluate insulin production.

Several health organizations, including the American Diabetes Association (ADA) and the U. S. Preventive Services Task Force (USPSTF), recommend diabetes screening when a person is age 45 or older or when a person of any age has risk factors. Examples of risk factors include:

• Overweight, obese, or physically inactive
• A close (first degree) relative with diabetes
• A woman who delivered a baby weighing more than 9 pounds or with a history of gestational diabetes
• A woman with polycystic ovarian syndrome
• High-risk race or ethnicity such as African American, Latino, Native American, Asian American, Pacific Islander
• High blood pressure (hypertension) or taking medication for high blood pressure
• Low HDL cholesterol level (less than 35 mg/dL or 0.90 mmol/L) and/or a high triglyceride level (more than 250 mg/dL or 2.82 mmol/L)
• A1c equal to or above 5.7%
• Prediabetes identified by previous testing
• History of cardiovascular disease (CVD)

If the screening test result is within normal limits, the ADA recommends retesting within 3 years, while the USPSTF recommends yearly testing. People with prediabetes may be monitored with annual testing.

See the screening articles for Children, Teens, Young Adults, Adults and Adults 50 and Up for additional details.

A blood glucose test may also be ordered when someone has signs and symptoms of high blood glucose (hyperglycemia), such as:

• Increased thirst, usually with frequent urination
• Fatigue
• Blurred vision
• Slow-healing wounds or infections

or symptoms of low blood glucose (hypoglycemia), such as:

• Sweating
• Hunger
• Trembling
• Anxiety
• Confusion
• Blurred vision

Diabetics are often required to self-check their glucose, up to several times a day, to monitor glucose levels and to determine treatment options as prescribed by their health practitioner. The healthcare provider may order blood glucose levels periodically in conjunction with other tests such as A1c to monitor glucose control over time.

Pregnant women are usually screened for gestational diabetes between their 24th and 28th week of pregnancy, unless they have early symptoms or have had gestational diabetes with a previous pregnancy. A woman may be tested earlier in her pregnancy if she is at risk of type 2 diabetes (overt diabetes), says the ADA. When a woman has type 1, type 2 or gestational diabetes, her health practitioner will usually order glucose levels throughout the rest of her pregnancy and after delivery to monitor her condition.

Blood Glucose

High levels of glucose most frequently indicate diabetes, but many other diseases and conditions can also cause elevated blood glucose.

In a person with signs and symptoms of diabetes or hyperglycemia, a non-fasting glucose level (random blood sample) that is equal to or greater than 200 mg/dL (11.1 mmol/L) indicates diabetes.

The following information summarizes the meaning of other test results.

Fasting Blood Glucose

2-Hour Oral Glucose Tolerance Test (OGTT)

Gestational Diabetes One-Step Approach (as one option recommended by the ADA)

Gestational Diabetes Two-Step Approach (as currently recommended by ACOG and as one option from the ADA): Step One

*Some experts recommend a cutoff of 130 mg/dL (7.2 mmol/L) because that identifies 90% of women with gestational diabetes, compared to 80% identified using the threshold of 140 mg/dL (7.8 mmol/L). ACOG recommends a lower threshold of 135 mg/dL (7.5 mmol/L) in high-risk ethnic groups with higher prevalence of gestational diabetes.

Gestational Diabetes Two-Step Approach (as currently recommended by ACOG and as one option from the ADA): Step Two

**Some labs may use different numbers.

Some other diseases and conditions that can result in an elevated blood glucose level include:

• Acromegaly
• Acute stress (response to trauma, heart attack, and stroke for instance)
• Chronic kidney disease
• Cushing syndrome
• Excessive consumption of food 
• Hyperthyroidism
• Pancreatic cancer
• Pancreatitis

A low level of glucose may indicate hypoglycemia, a condition characterized by a drop in blood glucose to a level where first it causes nervous system symptoms (sweating, palpitations, hunger, trembling, and anxiety), then begins to affect the brain (causing confusion, hallucinations, blurred vision, and sometimes even coma and death). A diagnosis of hypoglycemia uses three criteria known as the Whipple triad. (See the Common Questions section.)

A low blood glucose level (hypoglycemia) may be seen with:

• Adrenal insufficiency
• Drinking excessive alcohol
• Severe liver disease
• Hypopituitarism
• Hypothyroidism
• Severe infections
• Severe heart failure
• Chronic kidney (renal) failure
• Insulin overdose
• Tumors that produce insulin (insulinomas)
• Starvation
• Deliberate use of glucose-lowering products

Urine Glucose

Low to undetectable urine glucose results are considered normal. Any condition that raises blood glucose such as diabetes or the other conditions listed above also has the potential to elevate the concentration of glucose in the urine.

Increased urine glucose may be seen with medications, such as estrogens and chloral hydrate, and with some forms of kidney disease. Some people naturally leak glucose in their urine when blood levels are normal. Some medications used to treat diabetes work by increasing the elimination of glucose in the urine.

Extreme stress can cause a temporary rise in blood glucose. This can be a result of, for example, trauma, surgery, heart attack or stroke.

Drugs, including corticosteroids, tricyclic antidepressants, diuretics, epinephrine, estrogens (birth control pills and hormone replacement), lithium, phenytoin, and salicylates, can increase glucose levels, while drugs such as acetaminophen and anabolic steroids can decrease levels.